It is well known in the art that hydrocarbons may be prepared by means of the Fischer-Tropsch synthesis, in which a mixture of carbon monoxide and hydrogen is contacted at elevated temperature and pressure with a suitable catalyst. Recently, it has been found that certain catalysts active in the Fischer-Tropsch synthesis are highly selective in the preparation of hydrocarbons having high molecular weights, in particular paraffinic hydrocarbons. Fischer-Tropsch catalysts comprising cobalt as the catalytically active metal have been found to be particularly selective in the preparation of the aforementioned paraffinic hydrocarbons.
In the preparation of commercial products from the hydrocarbon effluent of the Fischer-Tropsch synthesis, it is most convenient to separate and refine the components of the effluent by applying distillation. The lower molecular weight components of the hydrocarbon effluent may be separated and refined by means of conventional distillation techniques operated at atmospheric or super-atmospheric pressures. However, the higher molecular weight components of the hydrocarbon effluent may be subject to thermal degradation at the high temperatures encountered in conventional atmospheric distillation operations. To avoid such degradation occurring, it is necessary to apply vacuum distillation techniques in the separation and refining of these higher molecular weight components.
A variety of vacuum distillation techniques and apparatus are known for separating and refining thermally sensitive materials, for example the operation under vacuum of conventional distillation columns as practiced in the conventional refining of crude oil. In addition, a number of specific vacuum distillation techniques have been developed, for example the short-path vacuum distillation techniques.
More recently, the use of a specific distillation apparatus, namely the wiped film evaporator, has been proposed for use in refining the thermally sensitive, high molecular weight hydrocarbons recovered from conventional crude oil refining operations. Thus, U.S. Pat. No. 5,032,249 discloses a process in which a petroleum wax, in particular a heavy intermediate petroleum wax, is separated into two fractions in a wiped film evaporator to provide a lower boiling fraction having a narrow melting range and a higher boiling fraction having a wider melting range.
It has now been found that wiped film evaporators may be advantageously applied in the separation and refining of the hydrocarbon product of a Fischer-Tropsch synthesis, in particular the higher molecular weight hydrocarbon components of the product.